This invention relates to electrodeless discharge lamps and more particularly to an improved design for such a lamp which yields higher efficiency, lower thermal loading and reduced radio interference as compared to lamps found in the prior art.
An electrodeless discharge lamp is described in U.S. Pat. No. 4,010,400 issued to Donald D. Hollister on Mar. 1, 1977. The lamp of that patent includes a sealed envelope, an ionizable medium within the envelope and a coil of wire wrapped around a non-magnetic core and positioned adjacent the envelope in close physical proximity to the ionizable medium to supply radio frequency (RF) energy to the medium. The ionizable medium emits radiant energy when subjected to the radio frequency field.
The lamp disclosed in the Hollister patent has the general overall shape of a conventional incandescent lamp, the coil being positioned in an open cylindrical cavity which extends through part of the distance of the envelope. This design has several resultant disadvantages. First, it does not have an optimum shape for discharge efficiency nor for coupling of the radio frequency energy to the ionizable medium. Additionally, there is a relatively high amount of thermal loading, i.e. a large amount of heat is generated inside of the lamp envelope. Although the frequency of the RF energy is chosen so that the base frequency and several higher harmonics do not interfere with FCC allotted broadcast frequencies, the energy from the high frequency coil produces a substantial amount of radio frequency interference within the immediate environment of the home or office. This can cause objectionable local radio interference with radio, T.V., microwave ovens, and the human body.
U.S. Pat. No. 3,521,120 issued to J. M. Anderson on July 21, 1970 shows an electrodeless fluorescent lamp having a hermetically sealed toroidal envelope containing an ionizable medium with the envelope surrounding a radio frequency coil and the ionizable medium is activated by the coil. This patent does not teach a design which maintains the RF field within the discharge volume, nor does it teach an arrangement for cooling the toroidal envelope. It therefore has two of the disadvantages of the Hollister lamp, i.e. the problems of radio frequency interference and high thermal loading.
It is an object of the present invention to provide an electrodeless discharge lamp having a geometry optimized for discharge efficiency and radio frequency coupling.
A further object is to provide an electrodeless discharge lamp having increased cooling efficiency as compared to lamps of the prior art.
Another object is to provide an electrodeless discharge lamp which substantially reduces radio frequency interference.
An additional object is to provide an electrodeless discharge lamp in which the heat distribution is more uniform than that found in lamps of the prior art.
Still a further object is to provide an electrodeless discharge lamp which is cooled by convection.
In accordance with the invention, an electrodeless discharge lamp is provided which utilizes an envelope having a toroidal shape. A toroid is defined as any planar shape which is rotated about an axis in the same plane, the axis not intersecting the planar shape. The envelope is hollow and is filled with an ionizable medium which is capable of emitting radiant energy when subjected to and ionized by the energy of a radio frequency field. In the preferred embodiment, transparent windings are coated on the interior, top and exterior surfaces of the envelope. The windings on the top and exterior surfaces confine the radio frequency field almost principally to within the toroid, thereby substantially eliminating radio interference while producing a more efficient coupling of radio frequency energy to the discharge. The free space near the bottom interior of the envelope is used to mount and cool the electronics needed to drive the radio frequency windings and provides convection to the interior of the toroid.
The foregoing brief description as well as further objects, features and advantages of the present invention are best appreciated by reading the following detailed description of several preferred embodiments in accordance with the invention while referring to the accompanying drawings .